Automatic Turn-off Relay

Parts List:
R1,R3 = 10K                     Q1 = 2N3904, or equivalent
   R2 = 680K (see text)        IC1 = 4001, or equivalent
R4,R5 = 6K8               D1,D2,D3 = 1N4001, or equivalent
   C1 = see text                Ry = Relay, 12V
   C2 = 0.1uF, ceramic

The above circuit is similar to the "Delayed Turn-on" type but now configured as an automatic turn-off type. The diagram shows how the circuit function can be reversed so that the relay turns on when power is applied but turns off again automatically after a preset delay. This response is obtained by modifying the relay-driving stage for an NPN transistor like the 2N3904.

It is worth noting again that the diagram provides a time delay of about 0.5 seconds for every microfarad in the value of capacitor C1. This permits delays of up to several minutes. If desired, the delay periods can be made variable by replacing resistor R2 with a fixed and variable resistor in series whose nominal values are approximately equal of the total value of R2 (680K).